Broadband internet has become ubiquitous throughout most of the developed world. Countries and states are ranked based on the fastest internet access available to citizens. People everywhere clamor for the newest gigabit internet connections to become available to them. Faster internet speeds allow real-time video and audio communication across vast distances, streaming of live or pre-produced video and audio content, downloading and installation of applications quickly and easily over the internet, and transmitting of large amounts of data very quickly for many other useful endeavors.
Fiber optic technology is used extensively to transmit the signals for internet access between a service provider and the homes and offices of consumers. Passive optical networks (PONS) are used in many instances to deliver high speed internet and other data capabilities to end users. FIG. 1 illustrates a typical PON topology as PON 10. PON 10 includes an optical line terminal (OLT) 12 connected to an optical beam splitter 14 via fiber optic cable 16. Splitter 14 outputs an optical signal from OLT 12 to a plurality of optical network units (ONUs) 21-24 via fiber optic cables 26-29. Splitter 14 also combines optical signals from ONUs 21-24 into a single signal to OLT 12. While four ONUs 21-24 are illustrated, more or less ONUs are connected to an individual OLT in other embodiments. A single OLT will commonly have a 32-way splitter to connect to 32 different ONUs. A splitter can also be used to connect 64 or more ONUs to a single OLT.
The equipment for OLT 12 is generally located at a building of an internet service provider (ISP), and directly connected to the ISPs infrastructure through electrical signals. Splitter 14 is located on the premises of the ISP along with OLT 12, or is located remotely in an outside plant. The outside plant with splitter 14 is commonly located in a neighborhood, with each house in the neighborhood being an ONU. In other embodiments, splitter 14 is located in the demarcation room of an office building, where an ONU is dedicated to each company occupying a suite of the office building.
PON 10 utilizes wavelength division multiplexing (WDM) to transmit bidirectionally over a single optical cable. One wavelength of light is used to transmit from OLT 12 to ONUs 21-24, while a second wavelength of light is used to transmit from an ONU to the OLT. Commonly used wavelengths of light are 1,490 nanometer (nm) for downstream traffic and 1,310 nm for upstream traffic. Downstream traffic refers to transmission from OLT 12 to ONUs 21-24, and upstream traffic refers to transmission from an ONU to the OLT. Both wavelengths of light are transmitted along fiber-optic cables 16 and 26-29 without interfering with each other. A transmission from OLT 12 occurs at the same time, but at a different transmission frequency, as the OLT receives a transmission.
OLT 12 transmits to ONUs 21-24 in continuous mode (CM). OLT 12 is able to continuously send traffic on the designated frequency. ONUs 21-24 transmit in burst mode (BM). An ONU 21-24 sends a burst of data to OLT 12 after receiving a grant from the OLT. The burst of data is for a specified amount of time, and then the transmitting ONU must pause so that other ONUs are able to transmit another burst of data on the same frequency.
A transmission from OLT 12 is divided by splitter 14 and sent separately to each ONU 21-25. Every ONU connected to OLT 12 receives every packet of data transmitted by the OLT. Every downstream packet sent by OLT 12 is received by each ONU 21-24. Packets sent by OLT 12 are addressed to a specific ONU 21-24, so that the other ONUs ignore packets that are not addressed for them. Encryption is used in some embodiments to prevent an unaddressed ONU from reading data meant for another ONU. Some packets may be broadcast packets that are intended for and received by every ONU connected to OLT 12.
Conversely, splitter 14 combines the signals from each ONU 21-24 into a single optical signal to OLT 12. OLT 12 receives every upstream packet sent by all ONUs 21-24 on the single fiber optic cable 16. However, having multiple ONUs communicate on a single fiber to OLT 12 is problematic. When two different optical signals from two different ONUs are transmitted on a single cable at the same time, the signals interfere with each other. OLT 12 is unable to receive any of the signals when optical signals from multiple ONUs 21-24 overlap on fiber 16.
To solve the problem, OLT 12 is responsible for allocating upstream bandwidth to ONUs 21-24. OLT 12 sends a “grant” to an ONU which gives one of the ONUs 21-24 permission to use a defined interval of time for upstream transmission. The other ONUs, which did not receive a grant, are silent while the transmitting ONU sends a burst of data. After the transmitting ONU's allotted time interval is over, OLT 12 sends another grant to another ONU. OLT 12 continues to allocate upstream bandwidth by sending a grant to, and then receiving data from, each individual ONU 21-24.
Another issue arises in the reality that each ONU 21-24 is located in a random location remote from splitter 14. The optical signal from each individual ONU 21-24 travels a significantly different path along a respective fiber optic cable 26-29. The ONUs 21-24 are commonly different distances away from splitter 14. In one embodiment, splitter 14 is on the first floor of an office building, ONU 21 is also on the first floor, and ONU 22 is on the fifth floor and one hundred yards across the building. In another embodiment, ONU 23 is at a house nearby splitter 14, while ONU 24 is at a house a significant distance down the street. The variable distance of ONUs 21-24 from OLT 12 results in an optical signal at OLT 12 that is of an unpredictable magnitude and phase. Magnitude refers to the amplitude or power level of a signal.
The phases of burst mode packets received by OLT 12 are different from packet to packet, since ONUs 21-24 are not synchronized to transmit optical packets in phase, and the distance between the OLT and ONUs are random. The magnitude of each received packet is different due to the varying distance of each ONU 21-24 from OLT 12. The optical signal from an ONU attenuates more the farther the light wave has to travel to reach OLT 12 due to optical power loss in fiber optic cables 16 and 26-29. To synchronize the phases of received signals, OLT 12 uses burst mode clock and data recovery (BM-CDR) to generate a local clock with the same frequency and phase as the individual received optical packet. In addition, a burst mode amplifier, commonly a trans-impedance amplifier (TIA or TZ amplifier), is used by OLT 12 to normalize the magnitude of the received signal. Each burst received by OLT 12 begins with a predefined preamble that does not contain packet data. OLT 12 uses the preamble signal, usually about 40 nanoseconds (ns), to lock in the phase and magnitude of the received burst. The burst preamble lasts as little as 20 ns, and as long as 300 ns, in some embodiments. In other embodiments, a burst preamble lasts any amount of time required for the specific PON 10 implementation.
A burst mode amplifier uses an automatic gain control (AGC) circuit to detect the magnitude of an incoming signal and apply an amplification or attenuation as necessary for the signal to be within a useful working range for the circuitry of OLT 12. Analog AGC circuits are available that provide continuous control of the gain setting. For PON networks, the gain setting should be fixed for the entire data burst being received. However, an analog AGC circuit tends to wander, i.e., the gain setting floats up and down, during transmission. Changes in the gain setting during transmission can cause data errors.
Discrete-state AGC circuits are available that have digital control and hold the gain at one of a plurality of preset states. Each state represents a different gain value or level of an amplifier within the AGC. A discrete-state AGC has hysteresis built in to prevent the AGC from switching gain levels a second time immediately after a first gain state transition. The hysteresis of a discrete-state AGC means that the threshold level for increasing the gain level is lower than the threshold for returning back to the lower gain level. Therefore, once a gain level threshold is reached by the input signal, the power level of the input signal is not near another threshold. The hysteresis is helpful in reducing mid-burst level transitions. However, a discrete state AGC may still switch state mid-burst if the receiver input level for the data burst is sufficiently close to one of the internally generated gain state thresholds.
The input signal power level will drift slightly during transmission of a burst due to differing spectral content of the burst preamble versus the burst payload. That is, the specific data in the payload may result in a signal that has a slightly different power level relative to the preamble. In some embodiments, on-off keying is used. On-off keying modulates a carrier wave by turning the carrier wave signal on and off. The on-time and off-time of the carrier wave represent digital ones and zeros, respectively. A burst preamble may be a square wave, where the power of the signal is almost exactly halfway between zero and the full carrier wave power. On the other hand, the payload data may include more ones than zeros, or more zeros than ones, which results in an average power level that drifts closer to zero or closer to the full carrier wave power level. Amplitude shift keying, or other keying protocols, may be used, and similarly may result in a drifting average power level.
While the average power level drifts over the course of a data burst, the actual amplitude of an optical signal received by OLT 12 does not generally change significantly during a burst. Once the gain of an AGC circuit is set during a burst preamble, the electrical signal magnitude within OLT 12 will remain steady, even though the average power level floats up or down. If the power level drifts across a nearby threshold, an unnecessary and disruptive gain state change occurs. A gain level change in the payload of a data burst from an ONU 21-24 to OLT 12 will significantly change the magnitude of the electrical signal received by the circuitry of the OLT. The sudden change in electrical signal magnitude will commonly result in data traffic errors. Current technology is insufficient to adequately prevent AGC gain state transitions during reception of a data burst payload.